The invention provides a process for producing a composite material by press molding lignocellulose-containing material glued with a binder in which the cold bonding strength of the lignocellulose-containing material is increased by using a specific type of binder and composite articles made by this process.
Press molded materials as finished products made from lignocellulose raw materials such as wood chips, wood fibers or veneers and binders or glues are manufactured by compression at different temperatures. Such molded materials play a substantial role as high-quality materials for the furniture, construction and packaging sectors.
Important binders for the production of these products are amino resins, phenolic resins and binders based on polyisocyanates from the diphenylmethane series (pMDI).
The disadvantage of the known pMDI binders is the poor cold bonding strength of the glued products. This becomes obvious at the edge regions when the glued cold prepressed moldings are moved without lateral support during the manufacturing process. Disintegration then takes place in the edge region so the molding then has to be trimmed back to the position of the broken edge.
A variety of suggestions has been made to solve this problem. According to EP-A 352 558, the cold bonding strength is improved by using polyisocyanates, compounds with at least two hydrogen atoms which can react with isocyanates and alkylene carbonates. EP-A 93 357 describes prepolymers based on urethane-modified diphenylmethane diisocyanates as binders. WO 99/19141 discloses binders made from diphenylmethane diisocyanates and isocyanate-reactive polymers for the production of plywood by veneer press molding.